This thread (and the Republican version) really says a lot about the two parties, and their strengths and weaknesses. As I understand it, it isn’t that difficult to create districts in California, New York and Illinois that are 95 % Obama (or maybe more). You usually don’t get close to those numbers for Romney or McCain, even in places like Alabama, Texas and Georgia. The almost monolithically Democratic cities are able to completely drown out Republican strengths in rural and exurban areas. That is of course an advantage for the Democrats.
The downside is of course that the Democratic core voters are locked into a few Congressional districts. That means that it isn’t that difficult to draw Republican districts in the suburbs and the countryside. In that respect, the current Republican coalition is better suited to control the House. Presidential Elections are of course a different story.
I would agree with the observation and add that this represents the fundamental shift from 40 years ago. At that time the GOP was better suited to win the White House while the Dems had a lock on the US House. Back then the cities were not as monolithic but there were significant rural areas that still voted for conservative New Deal Dems, especially in the South. The Dems now principally represent the basket of issues that are important to people who live in relatively dense diverse environments (ie cities), as opposed to the New Deal coalition which was primarily focused on issues affecting the working class and poor regardless of region.
The Democrats have always done well among low income voters. It goes without saying that you find a lot of these voters in the cities (at least in parts of the city). In recent years you have also seen the Democrats do well with high income (and highly educated) voters. You also find a lot of them in some big cities. This makes for a strong coalition. Another aspect is of course that many urban areas are very racially diverse. I assume that a lot of the white people that live there are also more liberal and less religious. That makes the large cities of the country very barren for the Republicans.
